An environment-friendly vehicle, such as a pure electric vehicle (EV), a hybrid electric vehicle (HEV) and a fuel cell vehicle (FCEV), which does not exhaust any gas at all, is driven by converting DC power charged in a main battery into 3-phase AC power a motor using an inverter to operate a motor and then, transferring the driving force of the motor to driving wheels.
In a motor system including a motor and an inverter installed in an environment-friendly vehicle, it has been known that the noise is reduced when a switching frequency of the inverter increases and the inverter efficiency and the fuel efficiency are enhanced when the switching frequency is decreased.
A single sampling scheme may be applied for reducing an inverter noise sensitively felt by a driver or a passenger. According to the single sampling scheme, a sampling frequency for obtaining information about a sensing current and a motor angle to control the inverter is set to be equal to the switching frequency of the inverter after the switching frequency is set high and fixed at it.
Although Noise, Vibration and Harshness (NVH) performance is improved when a base switching frequency is set high and fixed in all driving modes to reduce an inverter noise, the switching loss is increased and the inverter efficiency is deteriorated.
Thus, taking into consideration NVH performance, EMC performance, an inverter switching loss and control stability, a need exists to control the inverter switching frequency and the sampling frequency through a variable switching in a state that an environment-friendly vehicle is in a hill hold situation.
Unlike the environment-friendly vehicle, since an idle torque of an engine is transferred to a torque converter and a transmission even in the state that an accelerator pedal and a brake pedal of a vehicle are released to maintain the vehicle in a low-speed state, the vehicle may easily move on an inclined road such as a hill, so that the vehicle may be prevented from rolling backward in a hill hold situation.
In contrast, a hill hold function, which prevents a vehicle from rolling backward on an ascent road, may be implemented because an inverter controls the torque of a driving motor in an environment-friendly vehicle.
A method of determining whether an environment-friendly vehicle is in a hill hold section will be described as follows.
According to the related art, since three variables, i.e., a command torque, a driving motor speed and output information, for a motor are utilized for determining a hill hold section, after a hill hold condition is set for the three variables, values of the three variables are inputted to the condition to determine whether the vehicle is in a hill hold section.
Further, according to the related art, in order to prevent a vehicle from rolling backward and improve NVH performance of a vehicle for a switching noise of an inverter, the inverter does not operate at a base switching frequency, but performs a variable switching operation.
The inverter switching frequency is controlled through the variable switching operation to be higher than the base switching frequency, such that the switching noise sensitively felt by a driver and a passenger may be reduced.
In addition, the number of sampling, which is a control variable (current of a driving motor) for controlling a motor, is maintained corresponding to that in a base frequency band, so that the control accuracy is also maintained similarly to the base switching frequency control.
However, the related art has the following problems in the method of determining a hill hold section.
As described above, since the variables, such as the command torque, the driving motor speed and output information are utilized for determining a hill hold section in the related art, variable values may be inputted to the input condition of the variables for determining a hill hold condition even though the vehicle is not in a hill hold section.
That is, since the method of determining a hill hold section according to the related art is not accurate, variable values may be inputted to the input condition of the variables for determining a hill hold condition even though the vehicle is not in a hill hold section.
Thus, even though a vehicle is not in a hill hold section, the variable switching operation is performed so that the switching frequency increases. As the switching frequency increases, the inverter switching loss occurs and at the same time, the inverter switching efficiency is deteriorated.